The invention relates to an amplifier arrangement comprising a first amplifier for amplifying an input signal, the first amplifier comprising a first terminal for carrying a boosting voltage, which terminal is coupled to a first supply voltage terminal via a unidirectional element to receive a first supply voltage, a second terminal, which is coupled to a second supply voltage terminal to receive a second supply voltage, a first input terminal for receiving the input signal, and a first output terminal for supplying a first output signal superimposed on a specific direct voltage level, and a second amplifier for varying the boosting voltage on the first terminal depending upon the input signal, the second amplifier comprising a third and a fourth terminal, which are coupled to the first and the second supply voltage terminal respectively, a second input terminal for receiving a measure of the input signal and a second output terminal for supplying a second output signal, which output terminal is coupled to the first terminal by means of a capacitor.
Such an amplifier arrangement is suitable for boosting the output power of an amplifier operating with a small voltage difference between the two supply voltage terminals, as employed, for example, in car radios.
Such an amplifier arrangement is known from U.S. Pat. No. 4,752,747. In this known arrangement a similar input signal is applied to both the first and the second input terminal. This input signal is amplified by the first amplifier, the amplified signal appearing on the first output terminal, which is coupled to a load. However, the input signal is amplified by the second amplifier only if it exceeds a threshold value, the amplifier signal then being available on the second output terminal. Since the second output terminal is coupled to the first terminal via a capacitor, the boosting voltage on the first terminal will track the variations of the second output signal. In relation to the second supply voltage this boosting voltage determines the voltage difference at which the first amplifier is operated. The coupling of the first terminal to the first supply voltage terminal via the uni-directional element imposes a minimum value on the boosting voltage, which value is substantially equal to the first supply voltage. The boosting voltage is equal to said minimum value if the input signal does not exceed the threshold value and varies if the first output signal tends to be limited by the first supply voltage and the input signal consequently exceeds the threshold value. The boosting voltage can then become equal to at the most two times the voltage difference between the first and the second supply voltage terminal. This results in the maximum output power being quadrupled. In order to reach this maximum output power the direct voltage level on the first output terminal should be equal to the first supply voltage, so that the first output signal is centered relative to the maximum output voltage swing. However, for comparatively small input signals such a direct voltage level has the disadvantage that the output signal tends to be limited unnecessarily by the first supply voltage during on an average half the time and that, consequently, boosting voltage variations occur, attend with a comparatively high distortion. This can be reduced by another setting of the direct voltage level on the first terminal but this results in a reduced maximum output power owing to the off-centered position relative to the output voltage swing.